JP2017164788A - Friction stir spot welding method and friction stir spot welding apparatus - Google Patents

Abstract

An object of the present invention is to suppress a decrease in strength of a bonded portion when a workpiece having a protective layer is bonded by double-acting friction stir spot bonding. A friction stir spot welding method according to an aspect of the present invention includes a press-fitting step of press-fitting a shoulder from a first workpiece side toward a second workpiece while rotating a rotary tool, and a press-fitting step. And a backfilling step of backfilling the stirring material into a press-fitting hole generated by press-fitting the shoulder, and when the backfilling step is completed, the components of the protective layer In the press-fitting step, the shoulder is press-fitted at least 1 mm from the boundary between the first workpiece and the second workpiece to the second workpiece side so as to be concentrated in the central portion. [Selection] Figure 5

Description

  The present invention relates to a friction stir spot welding method and a friction stir spot welding apparatus.

  Friction spot joining (FSJ) is expected as an alternative to spot welding joining and rivet joining. In the friction stir spot welding, a rotating tool is press-fitted into the stacked workpieces, and both the workpieces are partially softened by frictional heat and stirred. Friction stir spot welding includes single-acting friction stir spot welding using a rotating tool with a shoulder and pin fixed, and double-acting friction stir spot welding using a rotating tool in which the shoulder and pin move relative to each other. is there. Of these, in double-acting friction stir spot welding, the backfilling process is performed after the press-fitting process, and the stir material is backfilled in the holes (press-fit holes) generated by press-fitting the rotary tool, so the marks due to the joining are conspicuous. It can be eliminated (see Patent Document 1).

JP 2012-196682 A

  By the way, a protective layer made of a material different from the base material may be formed on the surface of the object to be bonded by surface treatment such as anodizing treatment or primer coating. When the workpiece having this protective layer is joined by double-acting friction stir spot welding, compared to the case where the workpiece having no protective layer is joined by double-acting friction stirring spot joining in the same manner, It has been found that the strength of the joint is reduced. Furthermore, the inventors have discovered that the strength reduction of the joint is caused by the distribution of the protective layer component in the double-acting friction stir spot joining.

  The present invention has been made in view of the circumstances as described above, and friction that can suppress a decrease in strength of a joint when a workpiece having a protective layer is joined by double-acting friction stir spot joining. It aims at providing the stirring point joining method.

  A friction stir spot welding method according to an aspect of the present invention uses a rotary tool having a cylindrical shoulder and a columnar pin that can move inside the shoulder, and a first workpiece and a second workpiece. Is a double-acting friction stir spot joining method by friction stir spot joining, and a protective layer is formed on at least one joining surface of the first article to be joined and the second article to be joined. In the friction stir spot welding method, the shoulder is press-fitted from the first workpiece side toward the second workpiece while rotating the rotary tool, whereby the first workpiece and the first workpiece are joined. 2 The material of the object to be joined is partially stirred to form a stirring portion, and at that time, the press-fitting step for allowing the components of the protective layer to flow into the shoulder together with other stirring materials, and while rotating the rotary tool In the press-fitting process A backfilling step of backfilling the stirring material into a press-fitting hole caused by press-fitting the shoulder by retreating the shoulder at the same time as the stirring material flowing into the shoulder is pushed out of the shoulder by the pin; In the press-fitting step, the shoulder is connected to the first workpiece and the second workpiece so that the components of the protective layer are concentrated in the central portion of the stirring portion when the backfilling step is completed. 1 mm or more is press-fitted from the boundary of the bonded object to the second bonded object side.

  Here, the stirring portion is formed in the press-fitting step, but the component of the protective layer does not spread over the entire stirring portion. A component layer (residual layer) is formed. In the conventional friction stir spot joining method, the backfilling process is performed, so that the remaining layer moves to the vicinity of the position where the protective layer was present. In this case, when a force is applied to the joint portion, a crack is generated from the remaining layer, and it has been found through experiments by the inventors that this is a cause of a decrease in strength of the joint portion. On the other hand, according to the above method, in the backfilling process, the remaining layer can be concentrated on the central portion of the stirring portion, so that the remaining portion does not crack and suppresses the strength reduction of the joint portion. Can do.

  In the friction stir spot welding method described above, the surface pressure of the shoulder with respect to the stirrer in the backfilling step may be smaller than the surface pressure of the shoulder with respect to the stirrer in the press-fitting step.

  As described above, when the shoulder is press-fitted to a press-fitting depth of a certain level or more in the press-fitting process, not only the stirring part but also the material in the peripheral part thereof easily flows. Therefore, a phenomenon that the peripheral portion of the stirring unit rises due to the shoulder pressing the stirring unit in the backfilling process may occur. On the other hand, such a phenomenon can be prevented by reducing the surface pressure of the shoulder with respect to the stirring unit in the backfilling step as described above.

  Moreover, you may press the said shoulder to the said stirring part with the surface pressure of 90-175 MPa in the said backfilling process of said friction stir spot joining method.

  Thus, if the shoulder surface pressure with respect to the stirring part in the backfilling process is set to a relatively small value of 90 to 175 MPa, the bulge in the peripheral part of the stirring part that may be caused by pressing the stirring part against the shoulder is suppressed. Can do.

  In addition, the friction stir spot welding device according to one aspect of the present invention is a friction stir point that joins a first workpiece having a protective layer formed on at least one joining surface and a second workpiece by friction stir spot welding. A joining apparatus, comprising: a rotating tool having a cylindrical shoulder and a columnar pin movable within the shoulder; and a control unit for controlling the rotating tool, wherein the control unit includes the rotating tool. The material of the first workpiece and the second workpiece is partially agitated by press-fitting the shoulder from the first workpiece side toward the second workpiece while rotating. In this case, after the components of the protective layer are caused to flow into the shoulder together with other stirring materials, the stirring that has flowed into the shoulder in the press-fitting step while rotating the rotary tool. material The pin is pushed out of the shoulder by the pin, and at the same time, by retracting the shoulder, the stirring material is filled back into the press-fitting hole generated by press-fitting the shoulder, and the shoulder is formed when the stirring portion is formed. 1 mm or more is press-fitted into the second workpiece side from the boundary between the first workpiece and the second workpiece.

  According to the friction stir spot joining method described above, it is possible to suppress a decrease in strength of the joint portion when a workpiece having a protective layer is joined.

FIG. 1 is an overall view of a friction stir spot welding apparatus. FIG. 2 is a block diagram of a control system of the friction stir spot welding device. FIG. 3 is a conceptual diagram showing a press-fitting process of a conventional friction stir spot welding method. FIG. 4 is a conceptual diagram showing a backfilling process of a conventional friction stir spot welding method. FIG. 5 is a conceptual diagram showing a press-fitting process of the friction stir spot welding method according to the embodiment. FIG. 6 is a conceptual diagram showing a backfilling step of the friction stir spot welding method according to the embodiment.

<Jointing device>
First, a friction stir spot welding apparatus (hereinafter also simply referred to as “joining apparatus”) 100 used in the friction welding point joining method (hereinafter also simply referred to as “joining method”) according to the present embodiment will be described. FIG. 1 is a schematic view of a joining apparatus 100. FIG. 2 is a block diagram of a control system of the bonding apparatus 100. The joining device 100 is a device that joins the first workpiece 101 and the second workpiece 102 (hereinafter referred to as “the workpiece 103” together) by friction stir spot welding, and the main body 10, A tool unit 20 and a control unit 40 are provided.

  The main body 10 includes a frame 11 formed in a C shape, a backing 12 that is provided on a portion on one end side of the frame 11 (a lower portion in the drawing in FIG. 1) and supports the article 103 to be joined, and a backing 12. And a tool unit driving unit 13 provided at a portion on the other end side of the frame 11 (an upper portion on the paper surface of FIG. 1).

  The tool unit driving unit 13 can move the tool unit 20 in a direction toward the workpiece 103 (hereinafter referred to as “advance”) and move in a direction away from the workpiece 103 (hereinafter referred to as “retreat”). ). Moreover, the tool unit drive part 13 is provided with the tool unit pressure sensor 41 (refer FIG. 2). The tool unit drive unit 13 of the present embodiment is configured by a rack and pinion mechanism, but may be configured by a ball screw mechanism, a hydraulic actuator mechanism, or the like.

  The tool unit 20 includes a tool holding unit 21 connected to the tool unit driving unit 13 of the main body 10, a clamp 22 held by the tool holding unit 21, and also held by the tool holding unit 21 and positioned inside the clamp 22. And a rotating tool 23.

  The clamp 22 includes a cylindrical pressing member 24 and a biasing portion 25 that biases the pressing member 24 toward the workpiece 103. In addition, although the urging | biasing part 25 of this embodiment is comprised by the spring coil, you may comprise by a rack and pinion mechanism, a ball screw mechanism, a hydraulic actuator mechanism, etc.

  The rotary tool 23 is attached to the tool holding unit 21 via a rotary motor 26. Therefore, the rotating tool 23 rotates with respect to the tool holding unit 21 and the workpiece 103. The rotary tool 23 includes a shoulder holder 27 connected to the rotary motor 26, a shoulder 28 held by the shoulder holder 27, and a pin 29 located inside the shoulder 28.

  The shoulder 28 is located inside the pressing member 24 and has a cylindrical shape. The central axis of the shoulder 28 coincides with the rotational axis of the rotary tool 23. When performing friction stir spot welding, the shoulder 28 is press-fitted into the workpiece 103 while rotating.

  The pin 29 has a cylindrical shape. The central axis of the pin 29 coincides with the rotational axis of the rotary tool 23 and the central axis of the shoulder 28. The pin 29 is connected to the shoulder 28 via the pin driving unit 30. The pin driving unit 30 can move the pin 29 in the shoulder 28 along the axial direction. Note that the axial position of the pin 29 with respect to the workpiece 103 is determined by the amount of movement of the tool unit 20 and the amount of movement of the pin 29 with respect to the shoulder 28. Further, the pin 29 rotates together with the shoulder 28.

  The control unit 40 is configured by a CPU, a ROM, a RAM, and the like, and controls the entire joining apparatus 100 including the rotary tool 23.

  As shown in FIG. 2, the control unit 40 is electrically connected to the tool unit pressure sensor 41, and the tool unit 20 is attached to the workpiece 103 based on a measurement signal transmitted from the tool unit pressure sensor 41. The pressing force of the tool unit 20 when pressed (hereinafter referred to as “tool unit pressing force”) can be acquired. In addition, based on the acquired tool unit pressing force and the urging force of the clamp 22 (calculated from the movement amount of the tool unit 20), the pressing force of the shoulder 28 when the workpiece 103 is pressed (hereinafter referred to as "shoulder pressing force"). Can be calculated. The shoulder pressing force may be calculated by an external device (for example, an external PC) provided separately from the joining device 100, or may be calculated by the control unit 40.

  Further, the control unit 40 is electrically connected to the tool unit driving unit 13, the rotary motor 26, and the pin driving unit 30, and by transmitting a control signal to these devices, each of the tool units 20 can be arbitrarily set. The rotary tool 23 can be rotated at an arbitrary rotational speed (hereinafter referred to as “tool rotational speed”), and the pin 29 can be moved relative to the shoulder 28 with an arbitrary moving amount and It can be moved at a moving speed. The moving amount and moving speed of the shoulder 28 are equal to the moving amount and moving speed of the tool unit 20.

  The above is the description of the bonding apparatus 100. In the joining apparatus 100 described above, the clamp 22, the shoulder 28, and the pin 29 are configured to advance and retreat integrally, and the shoulder 28 and the pin 29 are configured to rotate integrally. ing. However, the clamp 22, shoulder 28, and pin 29 may be configured to move or rotate independently of each other. In FIG. 1, the first workpiece 101 and the second workpiece 102 are illustrated as flat members, but the first workpiece 101 and the second workpiece 102 are formed as flat members. Not limited.

<Conventional joining method>
Next, a conventional bonding method will be described before describing the bonding method according to the present embodiment. 3 and 4 are conceptual diagrams showing a conventional joining method. 3 and 4, only the shoulder 28, the pin 29, the first object 101, and the second object 102, which are the main parts, are illustrated (the same applies to FIGS. 5 and 6).

  Here, the 1st to-be-joined object 101 and the 2nd to-be-joined object 102 which are joining objects are aluminum alloys, Comprising: The surface (joint surface) which contact | connects the 2nd to-be-joined object 102 of the 1st to-be-joined object 101 is protected. It is assumed that the layer 104 is formed and the protective layer 104 is also formed on the surface (bonding surface) of the second bonded object 102 in contact with the first bonded object 101. The protective layer 104 includes an anodized layer, a coating layer, a cladding layer, a sealing layer, and the like.

  The conventional joining method includes a press-fitting step (FIG. 3) and a backfilling step (FIG. 4). Each process is performed by the control unit 40 transmitting control signals to the tool unit driving unit 13, the rotary motor 26, and the pin driving unit 30 to control the operation of the rotary tool 23.

  In the press-fitting process, the shoulder 28 is press-fitted into the workpiece 103. Specifically, as shown in FIG. 3, the shoulder 28 is advanced from the first workpiece 101 side toward the second workpiece 102 while rotating the rotary tool 23. At this time, the pin 29 is in a retracted state. The shoulder 28 is press-fitted from the boundary between the first workpiece 101 and the second workpiece 102 toward the second workpiece 102 to a position of 0.1 to 0.3 mm.

  By this press-fitting process, the materials of the first workpiece 101 and the second workpiece 102 are partially softened by frictional heat, and the softened material by the rotation of the shoulder 28 is stirred. As a result, a stirring portion 105 (shaded portion) made of the material (stirring material) stirred by the shoulder 28 is formed. At this time, the component of the base material of the first workpiece 101, the component of the base material of the second workpiece 102, and the component of the protective layer 104 flow into the shoulder 28.

  Here, the components of the protective layer 104 do not spread over the entire stirring portion 105 by the press-fitting step, but flow in a state of being hardened to some extent. Therefore, as shown in FIG. 3, a layer having a high component density of the protective layer 104, that is, a layer containing the component of the protective layer 104 as a main component (hereinafter referred to as “residual layer 106”) is formed. The remaining layer 106 flows into the shoulder 28 by the press-fitting process.

  In the above press-fitting process, the shoulder 28 is press-fitted into the workpiece 103. Therefore, when the shoulder 28 is moved back as it is and the stirring portion 105 is cooled and hardened, a press-fitting hole is formed in the portion where the shoulder 28 has been press-fitted. End up. Therefore, the backfilling step described below is performed subsequent to the press-fitting step so that the press-fitting hole is not formed in the stirring unit 105.

  In the backfilling process, the pin 29 is advanced with respect to the shoulder 28 while rotating the rotary tool 23. As a result, the stirring material flowing into the shoulder 28 is pushed out of the shoulder 28 by the pins 29. Then, the stirring material pushed out from the inside of the shoulder 28 applies a force to the end surface (the lower surface in FIG. 3) of the shoulder 28 in the traveling direction to retract the shoulder 28, and the portion into which the shoulder 28 has been press-fitted (press-fit) Flows into the hole). The above operation continues until the axial position of the surface of the pin 29 that contacts the workpiece 103 and the surface of the shoulder 28 that contacts the workpiece 103 coincide. Thereby, the press-fitting hole is backfilled with the stirring material, and the surface of the first workpiece 101 becomes flat.

  In the backfilling process, the shoulder 28 is retracted because the shoulder 28 is pushed up by the stirring material. Therefore, the shoulder 28 moves backward while pressing the workpiece 103. That is, the shoulder 28 moves in the direction indicated by the white arrow in FIG. 4, but presses the article 103 (stirring portion 105) in the direction opposite to the moving direction. The pressing force (shoulder pressing force) by the shoulder 28 at that time is about 200 MPa.

  When the backfilling process is completed, the first bonded object 101 and the second bonded object 102 are cooled and solidified in a state where the materials of the first bonded object 101 and the second bonded object 102 are partially stirred, thereby forming a cylindrical bonded part. As a result, the first article 101 and the second article 102 are joined.

  Here, as shown in FIG. 4, in the conventional joining method, when the stirring material is backfilled in the press-fitting hole in the backfilling process, the remaining layer 106 is pushed back to the vicinity of the position where the protective layer 104 was present. . Thus, when the joining of the first workpiece 101 and the second workpiece 102 is completed, the outer peripheral portion of the remaining layer 106 reaches the outer peripheral surface of the bonding portion (stirring portion 105) or the vicinity thereof, and the remaining layer The axial position of the outer peripheral portion 106 is close to the axial position of the boundary between the first workpiece 101 and the second workpiece 102. In this case, if a force is applied to the joint, cracks are likely to occur from the outer peripheral portion of the remaining layer 106. For this reason, when joining objects to be joined having a protective layer, the strength of the joint portion is reduced as compared to joining objects to be joined without a protective layer.

<Joining method according to the embodiment>
Next, the joining method according to this embodiment will be described. 5 and 6 are views showing a bonding method according to the present embodiment. Note that a protective layer 104 is formed on the bonding surfaces of the first workpiece 101 and the second workpiece 102.

  The joining method according to the present embodiment includes a press-fitting step (FIG. 5) and a backfilling step (FIG. 6), similarly to the conventional joining method. Each process is performed by the control unit 40 transmitting a control signal to the tool unit driving unit 13, the rotary motor 26, and the pin driving unit 30 to control the rotary tool 23.

  In the press-fitting process of the present embodiment, the shoulder 28 is advanced from the first workpiece 101 side toward the second workpiece 102 and press-fitted while rotating the rotary tool 23, as in the conventional press-fitting process. However, as shown in FIG. 5, the press-fit depth of the shoulder 28 into the workpiece 103 in this embodiment is larger than the press-fit depth in the conventional joining method (see FIG. 3). Specifically, in this embodiment, the shoulder 28 is press-fitted by 1 mm or more from the boundary between the first workpiece 101 and the second workpiece 102 to the second workpiece 102 side.

  As described above, when the press-fitting depth of the shoulder 28 is increased, the range of the stirring unit 105 is expanded toward the second workpiece 102 as compared with the case where the press-fitting depth is small. Therefore, among the stirring materials flowing into the shoulder 28, the amount of the base material component of the first bonded object 101 and the amount of the component of the protective layer 104 are substantially the same. Ingredients increase significantly.

  If the press-fitting of the shoulder 28 in the press-fitting process is stopped near the boundary between the first workpiece 101 and the second workpiece 102, the stirring material of the first workpiece 101 becomes the second workpiece 102 side. The first workpiece 101 and the second workpiece 102 are not sufficiently mixed. This tendency is particularly remarkable when impurities such as the protective layer 104 exist at the boundary between the first article 101 and the second article 102.

  Subsequently, in the backfilling process of the present embodiment, as in the conventional backfilling process, while rotating the rotary tool 23, the stirring material that has flowed into the shoulder 28 is pushed out of the shoulder 28 by the pins 29, The shoulder 28 is moved backward.

  As described above, in the joining method according to the present embodiment, the component of the base material of the second article to be joined flowing into the shoulder 28 in the press-fitting process is increased. Therefore, in the backfilling process, as the shoulder 28 moves backward, a large amount of the component of the base material of the second workpiece flows into the portion (press-fit hole) where the shoulder 28 is press-fitted. As a result, as shown in FIG. 6, when the backfilling process is completed, the components of the base material of the first workpiece 101 or the second workpiece 102 are present near the outer peripheral surface of the stirring portion 105 (joining portion). It spreads and the remaining layer 106 is confined inside the stirring unit 105. That is, the components of the protective layer 104 are concentrated in the central portion of the stirring unit 105. In other words, the ratio of the component of the protective layer 104 to the stirring material in the portion near the central axis of the stirring portion 105 is larger than that in the outer peripheral portion of the stirring portion 105.

  Therefore, the component of the protective layer 104 hardly reaches the outer peripheral portion of the stirring unit 105. Further, the axial position of the outer peripheral portion of the remaining layer 106 is also away from the axial position of the boundary between the first workpiece 101 and the second workpiece 102. Therefore, the joint part formed by the joining method according to the present embodiment has higher strength than the joint part formed by the conventional joining method.

  Note that the base material of the second workpiece 102 has many portions that are in direct contact with the shoulder 28 and has high comparative fluidity. Therefore, if the shoulder 28 is quickly retracted in the backfilling step, the component of the base material of the second workpiece 102 preferentially flows into the space (press-fit hole) generated by the retracting of the shoulder 28 and remains. The layer 106 can be covered with the base material of the second workpiece 102.

  Moreover, in this embodiment, since the press-fit depth of the shoulder 28 is large and the amount of heat due to friction increases, the material easily flows in a wider range. Therefore, if the shoulder pressing force in the backfilling process is set to the same level as the conventional joining method, the outer portion of the portion directly pressed against the shoulder 28 is raised against the urging force of the clamp 22, A large step is generated between the joint and its peripheral portion.

  Therefore, in the backfilling process of the present embodiment, the shoulder pressure is reduced as compared with the case of the conventional joining method. For example, in this embodiment, the surface pressure (shoulder pressure) of the shoulder 28 with respect to the stirring unit 105 in the backfilling process is made smaller than the surface pressure (shoulder pressure) of the shoulder 28 with respect to the stirring unit 105 in the press-fitting process. More specifically, the shoulder 28 is pressed against the stirring unit 105 with a surface pressure of 90 to 175 MPa. Thereby, the level | step difference between the junction part which may arise by a backfilling process, and its peripheral part can be made small or can be eliminated.

  Note that the portion where the bulge can occur in the backfilling step is a portion on the radially outer side of the stirring portion 105, that is, a portion corresponding to the clamp 22. Therefore, it is also conceivable to suppress the bulge by increasing the pressing force by the clamp 22. However, in this case, a mechanism for driving the clamp 22 is required separately, which may increase the size and complexity of the joining device 100. Therefore, the method for reducing the shoulder pressure in the backfilling process as described above is also effective in that the increase in size and complexity of the bonding apparatus 100 can be suppressed.

  The above is the description of the bonding method according to the present embodiment. In the bonding method described above, only the press-fitting step and the backfilling step have been described, but the bonding method may include steps other than the press-fitting step and the backfilling step. For example, the bonding method may include a step of contacting the surface of the first workpiece 101 while rotating the shoulder 28 and the pin 29 and heating the surface before the press-fitting step.

23 Rotary tool 28 Shoulder 29 Pin 40 Control unit 100 Friction stir spot welding apparatus 101 First workpiece 102 Second workpiece 104 Protective layer 105 Stirrer

Claims (4)

A double-acting friction stirrer that joins a first workpiece and a second workpiece by friction stir spot welding using a rotary tool having a cylindrical shoulder and a cylindrical pin that can move inside the shoulder. It is a point bonding method, and a protective layer is formed on at least one bonding surface of the first bonded object and the second bonded object,
The friction stir spot joining method is:
The shoulder is pressed into the second workpiece from the first workpiece side while rotating the rotary tool, so that the materials of the first workpiece and the second workpiece are partially obtained. To form a stirring portion, and in that case, a press-fitting step for allowing the components of the protective layer to flow into the shoulder together with other stirring materials,
While rotating the rotary tool, the stirring material that has flowed into the shoulder in the press-fitting step is pushed out of the shoulder by the pin, and at the same time, the shoulder is retracted to cause the shoulder to be press-fitted. A backfilling process for backfilling the stirring material into the press-fitting holes,
In the press-fitting step, the shoulder is a boundary between the first and second objects to be bonded so that the components of the protective layer are concentrated in the central portion of the stirring unit when the backfilling process is completed. Friction stir spot welding method in which 1 mm or more is press-fitted from the second side to the second workpiece.   The friction stir spot joining method according to claim 1, wherein a surface pressure of the shoulder with respect to the stirring portion in the backfilling step is made smaller than a surface pressure of the shoulder with respect to the stirring portion in the press-fitting step.   The friction stir spot joining method according to claim 1 or 2, wherein, in the backfilling step, the shoulder is pressed against the stirring portion with a surface pressure of 90 to 175 MPa. A friction stir spot welding device that joins a first workpiece and a second workpiece having a protective layer formed on at least one joining surface thereof by friction stir spot welding,
A rotating tool having a cylindrical shoulder and a columnar pin movable within the shoulder, and a control unit for controlling the rotating tool,
The controller is configured to press-fit the shoulder from the first workpiece side toward the second workpiece while rotating the rotary tool, whereby the first workpiece and the second workpiece are pressed. In the press-fitting step, the ingredients of the protective layer are made to flow into the shoulder together with other stirring materials, and then the rotating tool is rotated. The stirring material that has flowed into the shoulder is pushed out of the shoulder by the pin, and at the same time, by retracting the shoulder, the stirring material is refilled into the press-fitting hole generated by press-fitting the shoulder, and the press-fitting hole When the stirrer is formed, the shoulder is formed when the stirrer is formed so that the components of the protective layer are concentrated in the central portion of the stirrer. Conjugates and the second press-fitting or 1mm to the second object to be bonded side from the boundary of the object to be bonded, friction stir spot joining device.

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